Supramolecular chemistry of anions is increasingly addressing utilitarian problems, generally related to health, environment, or energy. The challenge with real-world problems related to anion recognition is that they typically involve competitive aqueous environments. One problem of special interest is sulfate separation from radioactive wastes. Sulfate is a problematic component of legacy nuclear wastes, particularly those in the U.S. Department of Energy (DOE) complex, as it interferes with the vitrification process selected for waste disposal, increases the volume of waste forms that must be produced and stored, and reduces their geologic performance (Lumetta 2004; Moyer et al. 2006).
For example, the DOE has approximately 92 Mgal of alkaline high-level tank wastes left over from Cold War era weapons production. These wastes consist of sludges and soluble salts, the latter being approximately 85% of the volume of the waste and containing half the radioactivity. After removal of the preponderance of the radioactivity, mostly 137Cs, the salt waste can be stored in a low-activity waste form, borosilicate glass or saltstone. Owing to the high cost of producing glass, there is a need to maximize waste loading and to minimize the volume of glass.
Sulfate at high concentrations in nuclear waste is a significant determinant of the volume of glass that must be produced for certain waste feeds to vitrification (Lumetta 2004; Moyer et al. 2006). Above a loading of approximately 0.8% SO3, separation of a sulfur-rich phase in the melter presents a significant electrode corrosion problem in vitrification, and corresponding insoluble, sulfur-rich phases in solidified glass degrade waste-form performance (Manara et al. 2007). Efficient removal of sulfate prior to vitrification therefore has the potential to benefit salt-waste processing by significantly reducing the mass of waste glass and processing costs (Hay et al. 2001).
In view of the potential cost savings to vitrification, economical methods for the removal of sulfate have been sought for more than a decade. This extremely challenging problem arises from the strongly hydrophilic nature of sulfate itself, as well as the high alkalinity, and the high concentration of competing anions such as nitrate, nitrite, carbonate, and aluminate found in the waste. Various techniques applicable at the industrial scale to sulfate removal from aqueous solutions include bioreduction, nanofiltration, reverse osmosis, adsorption, anion exchange, and crystallization (Darbi et al. 2003; Jong et al. 2003; Wang et al. 2007; Malaiyandi et al. 1981; Priyantha et al. 2000; Boukhalfa et al. 2007; Haghsheno et al. 2009; Boari et al. 1976; Tait et al. 2009; Benatti et al. 2009; Belcher et al. 1952). However, none have the requisite selectivity or are not operative under the conditions of high alkalinity and high ionic strength characteristic of nuclear waste.
Sulfate removal from aqueous solutions is already challenging enough due to the very strong hydration of this anion (ΔG°h=−1080 kJ/mol). The extreme ionic strength (>6 M) and alkalinity (pH 14) of the waste, as well as the high concentrations of competing anions (mainly NO3−, NO2−, OH−, and CO32−) further increase the complexity of the problem. Though examples of sulfate selective receptors have been previously reported (Ravikumar et al. 2012), none of them has been demonstrated to work in a viable binding-release cycle under the extremely demanding conditions found in nuclear wastes.
A promising approach to effective sulfate recognition and separation from competitive aqueous environments is to take inspiration from Nature's sulfate-binding protein (Pflugrath et al. 1985), and completely isolate the anion from the surrounding solvent by encapsulation inside structurally constrained cavities functionalized with complementary binding groups. Though such cryptand-like architectures (Kang et al., 2010) are often difficult to assemble via traditional organic synthesis, a more practical approach to cage receptors for anions via self-assembly from relatively simple building units has been recently demonstrated (Ballester 2010). Alternatively, self-assembly of crystalline solids that selectively include targeted anions upon crystallization can be effectively employed for anion separation (Custelcean 2010a; Custelcean 2009a; Custelcean et al. 2007). A distinct advantage of such crystalline ‘hosts’ is that the stiffer environment inside crystals may prevent the structural distortion of the anion-binding cavities and accommodation of competing anions, resulting in superior selectivity.
The tripodal ligand L1, consisting of a urea-functionalized tren scaffold (Raposo et al. 1995; Berrocal et al. 2000; Custelcean et al. 2005), has recently been reported to self-assemble with Mg(H2O)62+ cations and encapsulate sulfate upon crystallization from competitive aqueous solutions (Custelcean et al. 2008; Custelcean et al. 2010b). The rigid and highly complementary binding cavities of these crystalline capsules, comprising 12 urea hydrogen bonds to the sulfate, ensured exceptional sulfate selectivity based on shape, size, and charge discrimination. However, in spite of their excellent anion recognition abilities, these capsules have limited utility for sulfate separation from alkaline nuclear wastes, as they do not form under basic conditions (pH>10) due to Mg(OH)2 precipitation.
However, it has now been found that alkali metal cations, which are tolerant to highly basic conditions, provide a viable solution to the problem of sulfate separation from nuclear waste. Since lithium and potassium are virtually absent in nuclear wastes (Custelcean et al. 2009b), a sodium-based system, in particular, takes advantage of the abundance of sodium ions in the waste, and not only circumvents the need for adding external ionic components to the waste, but significantly decreases the solubility of the capsules through the common ion effect.
The anion-binding cavities in the monovalent sodium- and potassium-based crystalline capsules provide good shape and size recognition for sulfate as had previously been seen in sulfate separation selectivity in competitive crystallizations using the divalent Mg-based system (Custelcean et al. 2008; Custelcean et al. 2010a; Rajbanshi et al. 2011). However, previous systems (Mg and Li) were not suitable for use under highly alkaline, aqueous conditions so these new sodium and potassium systems meet this unfilled need and also provide a system useful under neutral and alkaline conditions in the absence of divalent cations, and particularly in the absence of magnesium cations.